Hydrogen gas has been conventionally produced by electrolysis of water (H2O→H2+1/2O2) or steam reforming of hydrocarbon (CH4+H2O→3H2+CO) . However, when the electrolysis method is used, the energy efficiency is low. When the steam reforming process is used, there are problems such as a large burden on the facility and disposal of by-products. Thus, a process for producing hydrogen by reaction of metal and water has been examined.
Typical examples of the metal to be used for hydrogen production include Al and an Al alloy. For example, in Patent documents 1, 2, and 4, hydrogen gas is generated and trapped by the steps of immersing Al in Ga with a melting point of 30° C. or less or a Ga—In alloy melt, removing a surface oxide film, exposing an active surface, and contact-reacting Al with water (Al+3H2O→Al (OH)3+1.5H2) . Further, a method including the steps of cutting and processing an Al alloy immersed in water and exposing a fresh active surface (Patent document 3), friction and dynamic fracture associated with friction (Patent document 5), thermal shock by rapid heating and quenching (Patent document 6) are known.
When the generation of hydrogen gas is dependant on the reaction of the active surface of the Al alloy and water, the hydrogen gas yield is influenced by the size of the active surface. It is said that the bulk of the Al alloy does not contribute to a hydrogen gas generation reaction. Since the size of the bulk is overwhelmingly larger than that of the active surface, the hydrogen gas yield is significantly lower than the consumption of the Al alloy. It may remain within several percent of a theoretical value of 1.3 l/g·Al which is calculated from a reaction formula of Al+3H2O→Al(OH)3+1.5H2.
Further, it is reported that hydrogen gas is generated by contact-reacting metals having a low melting point, such as Ga, In, Sn, or Zn and the alloyed Al with water (Nonpatent document 1) . From the viewpoint that the hydrogen gas generation reaction is occurred by bringing dopants such as Ga, In, Sn, or Zn and the alloyed Al into contact with water, the process is advantageous as compared to a method depending on the reaction of an active surface which is newly formed by cutting and frictioning the surface of the Al alloy with water. However, the alloyed Al alloy contains expensive metals such as Ga and In and thus it is not suitable from a cost standpoint.
Further, it is reported that the hydrogen gas generation reaction is induced by bringing alloy powder produced by adding 0 to 30% by weight of Bi to Al by the mechanical alloying process using a ball mill into contact with water (Nonpatent document 2).    Patent document 1: U.S. Pat. No. 4358291    Patent document 2: U.S. Pat. No. 4745204    Patent document 3: Japanese Patent Application Laid-Open (JP-A) No. 2001-31401    Patent document 4: JP-A No. 2003-12301    Patent document 5: JP-A No. 2004-123517    Patent document 6: JP-A No. 2006-45004    Nonpatent document 1: O. V Kravchenko et al., Journal of Alloys and Compounds 397(2005). pp. 58-62    Nonpatent document2: M-Q. Fan et al., International Journal of Hydrogen Energy, Corrected Proof, Available online 12 Feb. 2007